From memory it's been a while since I've stripped an 600's corbs off: -
Pic 1, Large black T union in the centre is the vacuum, which should have a elbow with a filter built in and face back towards the airbox, it's possibly still in the hose. The 2 smaller black T unions either side should be pointing straight down (6 O'clock) to the float bowls, these are fuel return lines, brown T union is fuel feed and should be point straight up (12 O'clock)but slightly forward, Pics 2-3 are the heater lines and they look correct.
Before you put them back on, check all the water heater elbows are clear, as this is where they block